Despite the growing literature reporting increased risks of adverse birth outcomes for women highly exposed to outdoor air pollution during pregnancy, the most important questions remain unanswered;specifically, what are the relevant sources and types of pollutants, exposure periods, and underlying biological mechanisms? Here we propose the first pilot study of fetal growth and air pollution in the U.S. relying on a unique resource of detailed, prospectively-collected pregnancy data from the Behavior In Pregnancy Study (BIPS) conducted in Los Angeles, California between 1993-1996. We hypothesize that maternal exposure to traffic-related air pollutants during pregnancy results in reductions in fetal growth, and that part of these impacts are seen early in pregnancy and persist until birth. This hypothesis will be addressed by: (1) estimating prenatal exposures to nitrogen oxides (NOx) - as a marker of traffic-related air pollutants - for BIPS participants using CALINE4 air dispersion modeling approaches;(2) examining associations between CALINE4 model-based traffic exposures and fetal size throughout pregnancy using ultrasound measures of biparietal diameter, occipital frontal diameter, head circumference, femur length, and abdominal circumference taken at approximately 18-20, 28- 30 and 35-36 weeks gestation;(3) examining associations between prenatal traffic exposures and weight, length, head circumference, fetal growth ratio, ponderal index, and cephalization index at birth;and (4) examining associations between prenatal exposures to the more regionally distributed air pollutants O3 and PM10 using measurement data from existing government air monitoring stations to examine whether prenatal exposure to such pollutants is also associated with reductions in fetal growth, or whether air pollution impacts are specific to pollutants originating from traffic. Exposure estimates will be based on women's residential locations and generated for the time periods between fetal ultrasound measurements. Statistical analyses will take into account a number of potentially confounding risk factors for which information is available from the BIPS data (e.g., maternal race/ethnicity, education level and stature, marital status, parity, smoking, infections). We will also perform analyses stratified on whether women moved or worked during pregnancy since residence-based exposure estimates may be more misclassified for these subjects. Findings from this pilot work will help guide decisions about future adjunct study proposals examining air pollution and pregnancy as part of the National Children's Study activities in Southern California, specifically by pinpointing: (1) whether and which fetal biometry measures are important to collect during pregnancy;(2) whether and which periods during pregnancy are particularly susceptible to air pollution (e.g., very early or very late in pregnancy);and (3) which air pollutants and sources should be targeted for more detailed neighborhood and personal air monitoring in Los Angeles. PUBLIC HEALTH RELEVANCE: Perturbations of fetal development put children at risk for mortality and morbidity in early life. There is also now evidence that intrauterine growth restriction can impair health in adulthood as well. Since motor vehicle usage will continue to grow and degrade urban air quality in both developed and developing countries, information on how exposures to traffic pollutants impact fetal development is essential from a public health perspective.